Lithographic method and lithographic apparatus

ABSTRACT

A method includes exposing number of fields on a substrate, obtaining data about a field and correcting exposure of the field in subsequent exposures. The method includes defining one or more sub-fields of the field based on the obtained data. Data relating to each sub-field is processed to produce sub-field correction information. A subsequent exposure of the one or more sub-fields is corrected using the sub-field correction information. By controlling a lithographic apparatus by reference to data of a particular sub-field within a field, overlay error can be reduced or minimized for a critical feature, rather than being averaged over the whole field. By controlling a lithographic apparatus with reference to a sub-field rather than only the whole field, a residual error can be reduced in each sub-field.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of EP application 15158935.5 which wasfiled on Mar. 13, 2015 and which is incorporated herein in its entiretyby reference.

FIELD

The present invention relates to a method of controlling a lithographicprocess. In particular, the invention relates to a method for reducingoverlay errors on a substrate by processing data relating to sub-fieldsof a field. The invention further relates to lithographic apparatusconfigured for performing such methods, and to computer program productsfor use in controlling the lithographic apparatus to perform themethods.

BACKGROUND

A lithographic process is one that applies a desired pattern onto asubstrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a product pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate.Stepping and/or scanning movements can be involved, to repeat thepattern at successive target portions across the substrate. It is alsopossible to transfer the pattern from the patterning device to thesubstrate by imprinting the pattern onto the substrate. The pattern canbe transformed into functional product features by further processingsteps.

A key performance parameter of the lithographic process is the overlayerror. This error, often referred to simply as “overlay” is the error inplacing a product features in the correct position relative to featuresformed in previous layers. As product feature become all that muchsmaller, overlay specifications become ever tighter.

Currently the overlay error is controlled and corrected by means ofcorrection models described for example in US2013230797A1. Advancedprocess control techniques have been introduced in recent years and usemeasurements of metrology targets applied to substrates alongside theapplied device pattern. These targets allow overlay to be measured usinga high-throughput inspection apparatus such as a scatterometer, and themeasurements can be used to generate corrections that are fed back intothe lithographic apparatus when patterning subsequent substrates.Examples of advanced process control (APC) are described for example inUS2012008127A1. The inspection apparatus may be separate from thelithographic apparatus. Within the lithographic apparatus wafercorrection models are conventionally applied based on measurement ofoverlay targets provided on the substrate, the measurements being as apreliminary step of every patterning operation. The correction modelsnowadays include higher order models, to correct for non-lineardistortions of the wafer. The correction models may also be expanded totake into account other measurements and/or calculated effects such asthermal deformation during a patterning operation

While the use of higher order models may be able take into account moreeffects, however, such models require that more position measurementsare made. Further, higher order correction models require more computingpower and/or take more time to calculate. Thus, using an advancedcorrection model may in certain circumstances be feasible in theory, butmay not be economically viable in practice since it would negativelyinfluence throughput of the lithographic process (i.e. wafers per hour).Additionally, more advanced correction models may be of limited use, ifthe patterning apparatus itself does not provide control ofcorresponding parameters during patterning operations. Furthermore, evenadvanced correction models may not be sufficient or optimized to correctfor certain overlay errors.

SUMMARY

It is desirable to improve overlay control and correction potentialwithout adversely impacting throughput. It is further desirable thatsuch improvement can be obtained by using existing lithographic methodsand apparatuses. This enables existing lithographic apparatuses to beupgraded, thereby extending their effective lifetimes.

According to an aspect of the invention, there is provided alithographic method, the method comprising:

-   -   exposing a number of fields on a substrate;    -   obtaining data about a field;    -   defining a sub-field of the field based on the obtained data;    -   processing data relating to the sub-field to produce sub-field        correction information; and    -   correcting exposure of the sub-field using the sub-field        correction information.

In some embodiments, the data obtained is the fingerprint for the field.In a particular embodiment, the sub-field is a line of data points inthe fingerprint. The data may additionally or alternatively comprisetopography, layout, structure or simulation data.

In one embodiment, the lithographic method further comprises processingdata relating to a number of sub-fields to produce sub-field correctioninformation for each sub-field and correcting exposure of each sub-fieldusing correction information for that sub-field.

The invention further provides a lithographic apparatus for implementingthe method as set forth above.

The invention further provides a computer program product containing oneor more sequences of machine-readable instructions configured to controla lithographic apparatus to perform the method as set forth above.

The invention further provides a computer program product containing oneor more sequences of machine-readable instructions configured to controla lithographic apparatus to perform the method as set forth above,wherein the computer program product comprises a user interface.

These and further features and advantages of the invention will beapparent to the skilled reader from a consideration of the detaileddescription of examples that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 depicts a lithographic cell or cluster incorporating theapparatus of FIG. 1;

FIG. 3 illustrates schematically measurement and exposure processes inthe apparatus of FIG. 1, according to known practice and modified inaccordance with an embodiment of the present invention;

FIGS. 4 & 5 illustrate the principles of advanced alignment measurementsand wafer grid corrections applied in a lithographic apparatus of aproduction facility;

FIG. 6 is a flowchart of a process implementing one embodiment of thepresent invention;

FIG. 7 is schematic illustration of a field divided into sub-fields;

FIG. 8 illustrates schematically a wafer divided into a number offields, as well as different sub-fields divisions;

FIG. 9 is a flowchart of a sub-process of the embodiment of FIG. 6;

FIG. 10 is a schematic illustration of a principle of the process ofFIG. 6;

FIG. 11 is an example of the process of FIG. 6; and

FIG. 12 is a graph showing the relative motion stages determined by theprocess shown in FIG. 11.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Before describing embodiments of the invention in detail, it isinstructive to present an example environment in which embodiments ofthe present invention may be implemented.

FIG. 1 schematically depicts a lithographic apparatus. The apparatuscomprises:

-   -   an illumination system (illuminator) IL configured to condition        a radiation beam B (e.g. UV radiation or DUV radiation).    -   a support structure (e.g. a mask table) MT constructed to        support a patterning device (e.g. a mask) MA and connected to a        first positioner PM configured to accurately position the        patterning device in accordance with certain parameters;    -   a substrate table (e.g. a wafer table) WT constructed to hold a        substrate (e.g. a resist-coated wafer) W and connected to a        second positioner PW configured to accurately position the        substrate in accordance with certain parameters; and    -   a projection system (e.g. a refractive projection lens system)        PL configured to project a pattern imparted to the radiation        beam B by patterning device MA onto a target portion C (e.g.        comprising one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports, i.e. bears the weight of, the patterningdevice. It holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support structurecan use mechanical, vacuum, electrostatic or other clamping techniquesto hold the patterning device. The support structure may be a frame or atable, for example, which may be fixed or movable as required. Thesupport structure may ensure that the patterning device is at a desiredposition, for example with respect to the projection system. Any use ofthe terms “reticle” or “mask” herein may be considered synonymous withthe more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable minor array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system Immersion techniques are wellknown in the art for increasing the numerical aperture of projectionsystems. The term “immersion” as used herein does not mean that astructure, such as a substrate, must be submerged in liquid, but ratheronly means that liquid is located between the projection system and thesubstrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing minors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having traversed the mask MA, theradiation beam B passes through the projection system PL, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF (e.g. an interferometricdevice, linear encoder, 2-D encoder or capacitive sensor), the substratetable WT can be moved accurately, e.g. so as to position differenttarget portions C in the path of the radiation beam B. Similarly, thefirst positioner PM and another position sensor (which is not explicitlydepicted in FIG. 1) can be used to accurately position the mask MA withrespect to the path of the radiation beam B, e.g. after mechanicalretrieval from a mask library, or during a scan. In general, movement ofthe mask table MT may be realized with the aid of a long-stroke module(coarse positioning) and a short-stroke module (fine positioning), whichform part of the first positioner PM. Similarly, movement of thesubstrate table WT may be realized using a long-stroke module and ashort-stroke module, which form part of the second positioner PW. In thecase of a stepper (as opposed to a scanner) the mask table MT may beconnected to a short-stroke actuator only, or may be fixed. Mask MA andsubstrate W may be aligned using mask alignment marks M1, M2 andsubstrate alignment marks P1, P2. Although the substrate alignment marksas illustrated occupy dedicated target portions, they may be located inspaces between target portions (these are known as scribe-lane alignmentmarks). Similarly, in situations in which more than one die is providedon the mask MA, the mask alignment marks may be located between thedies.

The depicted apparatus could be used in at least one of the followingmodes:

-   -   1. In step mode, the mask table MT and the substrate table WT        are kept essentially stationary, while an entire pattern        imparted to the radiation beam is projected onto a target        portion C at one time (i.e. a single static exposure). The        substrate table WT is then shifted in the X and/or Y direction        so that a different target portion C can be exposed. In step        mode, the maximum size of the exposure field limits the size of        the target portion C imaged in a single static exposure.    -   2. In scan mode, the mask table MT and the substrate table WT        are scanned synchronously while a pattern imparted to the        radiation beam is projected onto a target portion C (i.e. a        single dynamic exposure). The velocity and direction of the        substrate table WT relative to the mask table MT may be        determined by the (de-)magnification and image reversal        characteristics of the projection system PL. In scan mode, the        maximum size of the exposure field limits the width (in the        non-scanning direction) of the target portion in a single        dynamic exposure, whereas the length of the scanning motion        determines the height (in the scanning direction) of the target        portion.    -   3. In another mode, the mask table MT is kept essentially        stationary holding a programmable patterning device, and the        substrate table WT is moved or scanned while a pattern imparted        to the radiation beam is projected onto a target portion C. In        this mode, generally a pulsed radiation source is employed and        the programmable patterning device is updated as required after        each movement of the substrate table WT or in between successive        radiation pulses during a scan. This mode of operation can be        readily applied to maskless lithography that utilizes        programmable patterning device, such as a programmable mirror        array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

As shown in FIG. 2, the lithographic apparatus LA forms part of alithographic cell LC, also sometimes referred to a lithocell or cluster,which also includes apparatus to perform pre- and post-exposureprocesses on a substrate. Conventionally these include spin coaters

SC to deposit resist layers, developers DE to develop exposed resist,chill plates CH and bake plates BK. A substrate handler, or robot, ROpicks up substrates from input/output ports I/O1, I/O2, moves thembetween the different process apparatus and delivers then to the loadingbay LB of the lithographic apparatus. These devices, which are oftencollectively referred to as the track, are under the control of a trackcontrol unit TCU which is itself controlled by the supervisory controlsystem SCS, which also controls the lithographic apparatus vialithography control unit LACU. Thus, the different apparatus can beoperated to maximize throughput and processing efficiency.

In order that the substrates that are exposed by the lithographicapparatus are exposed correctly and consistently, it is desirable toinspect exposed substrates to measure properties such as overlay errorsbetween subsequent layers, line thicknesses, critical dimensions (CD),etc. Accordingly a manufacturing facility in which lithocell LC islocated also includes metrology system MET which receives some or all ofthe substrates W that have been processed in the lithocell. Metrologyresults are provided directly or indirectly to the supervisory controlsystem SCS. If errors are detected, adjustments may be made to exposuresof subsequent substrates, especially if the inspection can be done soonand fast enough that other substrates of the same batch are still to beexposed. Also, already exposed substrates may be stripped and reworkedto improve yield, or discarded, thereby avoiding performing furtherprocessing on substrates that are known to be faulty. In a case whereonly some target portions of a substrate are faulty, further exposurescan be performed only on those target portions which are good.

Within metrology system MET, an inspection apparatus is used todetermine the properties of the substrates, and in particular, how theproperties of different substrates or different layers of the samesubstrate vary from layer to layer. The inspection apparatus may beintegrated into the lithographic apparatus LA or the lithocell LC or maybe a stand-alone device. To enable most rapid measurements, it isdesirable that the inspection apparatus measure properties in theexposed resist layer immediately after the exposure. However, the latentimage in the resist has a very low contrast—there is only a very smalldifference in refractive index between the parts of the resist whichhave been exposed to radiation and those which have not—and not allinspection apparatus have sufficient sensitivity to make usefulmeasurements of the latent image. Therefore measurements may be takenafter the post-exposure bake step (PEB) which is customarily the firststep carried out on exposed substrates and increases the contrastbetween exposed and unexposed parts of the resist. At this stage, theimage in the resist may be referred to as semi-latent. It is alsopossible to make measurements of the developed resist image—at whichpoint either the exposed or unexposed parts of the resist have beenremoved—or after a pattern transfer step such as etching. The latterpossibility limits the possibilities for rework of faulty substrates butmay still provide useful information.

FIG. 3 illustrates the steps to expose target portions (e.g. dies) on asubstrate W in the dual stage apparatus of FIG. 1.

On the left hand side within a dotted box are steps performed at ameasurement station MEA, while the right hand side shows steps performedat the exposure station EXP. From time to time, one of the substratetables WTa, WTb will be at the exposure station, while the other is atthe measurement station, as described above. For the purposes of thisdescription, it is assumed that a substrate W has already been loadedinto the exposure station. At step 300, a new substrate W′ is loaded tothe apparatus by a mechanism not shown. These two substrates W, W′ areprocessed in parallel in order to increase the throughput of thelithographic apparatus.

Referring initially to the newly-loaded substrate W′, this may be apreviously unprocessed substrate, prepared with a new photo resist forfirst time exposure in the apparatus. In general, however, thelithography process described will be merely one step in a series ofexposure and processing steps, so that substrate W′ has been throughthis apparatus and/or other lithography apparatuses, several timesalready, and may have subsequent processes to undergo as well.Particularly for the problem of improving overlay performance, the taskis to ensure that new patterns are applied in exactly the correctposition on a substrate that has already been subjected to one or morecycles of patterning and processing. These processing stepsprogressively introduce distortions in the substrate that must bemeasured and corrected for, to achieve satisfactory overlay performance.

The previous and/or subsequent patterning step may be performed in otherlithography apparatuses, as just mentioned, and may even be performed indifferent types of lithography apparatus. For example, some layers inthe device manufacturing process which are very demanding in parameterssuch as resolution and overlay may be performed in a more advancedlithography tool than other layers that are less demanding. Thereforesome layers may be exposed in an immersion type lithography tool, whileothers are exposed in a ‘dry’ tool. Some layers may be exposed in a toolworking at DUV wavelengths, while others are exposed using EUVwavelength radiation.

At 302, alignment measurements using the substrate marks P1 etc. andimage sensors (not shown) are used to measure and record alignment ofthe substrate relative to substrate table WTa/WTb. In addition, severalalignment marks across the substrate W′ will be measured using alignmentsensor AS. These measurements are used in one embodiment to establish a“wafer grid”, which maps very accurately the distribution of marksacross the substrate, including any distortion relative to a nominalrectangular grid.

At step 304, a map of wafer height (Z) against X-Y position is measuredalso using the level sensor LS. Conventionally, the height map is usedonly to achieve accurate focusing of the exposed pattern. As will beexplained further below, the present apparatus uses height map data alsoto supplement the alignment measurements.

When substrate W′ was loaded, recipe data 306 were received, definingthe exposures to be performed, and also properties of the wafer and thepatterns previously made and to be made upon it. To these recipe dataare added the measurements of wafer position, wafer grid and height mapthat were made at 302, 304, so that a complete set of recipe andmeasurement data 308 can be passed to the exposure station EXP. Themeasurements of alignment data for example comprise X and Y positions ofalignment targets formed in a fixed or nominally fixed relationship tothe product patterns that are the product of the lithographic process.These alignment data, taken just before exposure, are combined andinterpolated to provide parameters of a correction model. Theseparameters and the correction model will be used during the exposureoperation to correct positions of patterns applied in the currentlithographic step. A conventional correction model might comprise four,five or six parameters, together defining translation, rotation andscaling of the ‘ideal’ grid, in different dimensions. As describedfurther in US 2013230797A1, advanced models are known that use moreparameters.

At 310, wafers W′ and W are swapped, so that the measured substrate W′becomes the substrate W entering the exposure station EXP. In theexample apparatus of FIG. 1, this swapping is performed by exchangingthe supports WTa and WTb within the apparatus, so that the substrates W,W′ remain accurately clamped and positioned on those supports, topreserve relative alignment between the substrate tables and substratesthemselves. Accordingly, once the tables have been swapped, determiningthe relative position between projection system PS and substrate tableWTb (formerly WTa) is all that is necessary to make use of themeasurement information 302, 304 for the substrate W (formerly W′) incontrol of the exposure steps. At step 312, reticle alignment isperformed using the mask alignment marks M1, M2. In steps 314, 316, 318,scanning motions and radiation pulses are applied at successive targetlocations across the substrate W, in order to complete the exposure of anumber of patterns.

By using the alignment data and height map obtained at the measuringstation in the performance of the exposure steps, these patterns areaccurately aligned with respect to the desired locations, and, inparticular, with respect to features previously laid down on the samesubstrate. The exposed substrate, now labeled W″ is unloaded from theapparatus at step 320, to undergo etching or other processes, inaccordance with the exposed pattern.

Even when advanced correction models are used, errors inevitably remainin the overlay performance of the lithographic apparatus. An individuallithographic apparatus may also perform differently than other onesprocessing the same substrate. In order that the substrates that areexposed by the lithographic apparatus are exposed correctly andconsistently, it is desirable to inspect exposed substrates to measureperformance parameters such as overlay errors between subsequent layers,line thicknesses, critical dimensions (CD), etc.

An inspection apparatus is therefore used to determine the properties ofthe substrates independently of the alignment sensors AS, and inparticular, how the properties of different substrates or differentlayers of the same substrate vary from layer to layer. The inspectionapparatus (not shown in FIG. 2) may be integrated into the lithographicapparatus LA or the lithocell LC or may be a stand-alone device. It maybe a scatterometer, for example an angle-resolved scatterometer of thetime described in published US patent application US2006033921A1.

The inspection apparatus can also be used in an advanced process control(APC) system to calibrate individual lithographic apparatus and to allowdifferent tools to be used more interchangeably. Improvements to theapparatus's focus and overlay (layer-to-layer alignment) uniformity haverecently been achieved by the implementation of a stability module,leading to an optimized process window for a given feature size and chipapplication, enabling the continuation the creation of smaller, moreadvanced chips. The stability module in one embodiment automaticallyresets the system to a pre-defined baseline at regular intervals, forexample each day. More detail of lithography and metrology methodsincorporating the stability module can be found in US2012008127A1. Theknown example implements three main process control loops. The firstloop provides the local control of the lithography apparatus using thestability module and monitor wafers. The second (APC) loop is for localscanner control on-product (determining focus, dose, and overlay onproduct wafers).

The third control loop is to allow metrology integration into the second(APC) loop (e.g., for double patterning). All of these loops usemeasurements made by the inspection apparatus, in addition to themeasurements made in the during the actual patterning operations of FIG.3.

As explained above, the diagnostic methods and apparatus disclosedherein employ object data that is data measured from points distributedspatially over each product unit. In the example of a lithographicproduction facility where the product units are semiconductor substrates(wafers), a particularly interesting source of comprehensive object datais the set of measurements performed in the lithographic apparatus tocharacterize each wafer and the patterns previously deposited upon it.These measurements are used to obtain parameters for correction models,that are used in a new patterning step to control accurately thepositioning of patterns applied in relation to features already present

Standard intra-field and inter-field correction models have sixparameters (effectively three per direction X & Y) and in addition thereare more advanced correction models. On the other hand, for the mostdemanding processes currently in use and under development, to achievethe desired overlay performance requires more detailed corrections.While standard models might use fewer than ten parameters, advancedcorrection models typically use more than 15 parameters, or more than 20parameters.

FIGS. 4 & 5 illustrate the form of correction information that can beused to correct for wafer grid distortion as measured by the alignmentsensor AL on alignment marks (targets) 400 in a previous layer on wafer(substrate) W. Each target has a nominal position, defined usually inrelation to a regular, rectangular grid 402 with axes X and Y.Measurements of the real position 404 of each target reveal deviationsfrom the nominal grid. The alignment marks may be provided within deviceareas of the substrate, and/or they may be provided in so-called “scribelane” areas between device areas.

As illustrated in FIG. 5 the measured positions 404 of all the targetscan be processed numerically to set up a model of a wafer grid for thisparticular wafer. This correction model is used in the patterningoperation to control the position of the patterns applied to thesubstrate. FIG. 5(a) shows the measured positions 404 of all thetargets. A highlighted region 408 is also shown. FIG. 5(b) shows anexample wherein a standard correction model with six parameters is usedto model the wafer grid. The parameters of the modeled wafer grid 406are modified to fit the modeled wafer grid to the measured targets 404,which are shown for reference. Since the standard correction model onlyhas six parameters, it is not possible to fit the modeled wafer gridperfectly to all of the measured positions of the targets on the waferW. As can be seen in FIG. 5(b), the modeled wafer grid 406 is fittedclosely to the measured targets within the region of the highlightedarea 408. However, outside the highlighted area, the modeled wafer griddeviates from the measured grid. In other terms, the modeled wafer grid406 has been optimized for the highlighted area 408, to ensure that thedeviations inside the area are small. Thus, modeled wafer grids arenormally optimized for areas with critical components or products, whichrequire that the overlay error is small. Less critical products orcomponents can be placed outside the highlighted area. It is of courseto be noted that the position of the highlighted area in the presentexample is exemplary only, and that the modeled wafer grid can beoptimized for any appropriate location on the wafer. Of course, forcertain processes, certain area shapes are not possible. In such cases,the design layout of the substrate can be adjusted to make it easier toposition the critical components within an area with a particular shape

FIG. 5(c) shows the measured positions 404 of all the targets in amanner similar to FIG. 5(a), but without the highlighted area. In theexemplary modeled wafer grid 410 illustrated in FIG. 5(d), the straightlines of the nominal grid have become curves, indicating use of a higherorder (advanced) correction model. The use of a higher order correctionmodel allows the modeled wafer grid to be matched more closely to themeasured grid than the standard correction model. However, even in thiscase residual deviations (not shown) will remain in practice. Even whena higher order model is used, there can still be scope to definespecific areas as critical areas, and optimize the model to minimizedeviations in those areas. Since more advanced correction models havemore parameters, it is necessary to perform more measurements on awafer, which in turn requires more time for performing thesemeasurements. This reduces throughput of wafers in a productionsituation, which is not desirable.

It goes without saying that the distortions illustrated are exaggeratedcompared to the real situation. Alignment is a unique part of thelithographic process, because it is the correction mechanism able tocorrect for deviations (distortions) in each exposed wafer.

Certain components of the overlay on each substrate will be truly randomin nature. However, other components will be systematic in nature,whether their cause is known or not. Where similar substrates aresubject to similar patterns of overlay error, the patterns of error maybe referred to as “fingerprints” of the lithographic process. Overlayerrors can broadly be categorized into two distinct groups:

-   -   1) contributions which vary across an entire substrate, wafer        are known in the art as inter-field fingerprints.    -   2) contributions which vary similarly across each target portion        (field) of a substrate or wafer are known in the art as        intra-field fingerprints.

Advanced correction models can be applied to correct both theinter-field fingerprints and intra-field fingerprints. Each fingerprintmay have components due to different causes, e.g. a scanner may have afingerprint unique to itself, or an etching process may have aparticular fingerprint. All these components of inter-field fingerprintsand intra-field fingerprints combine into the error actually present ona given substrate.

However, while an advanced correction model may, for example, include20-30 parameters, scanners currently in use may not have actuators whichcorrespond to one or more of the parameters. Hence, only a subset of theentire set of parameters of the model can be used at any given time.Additionally, as the advanced models require many measurements, it isnot desirable to use these models in all situations, since the timerequired to perform the necessary measurements reduces throughput.

Overlay Error Sources and Reduction

Some of the main contributors to overlay errors include, but are notlimited to, the following:

-   -   scanner-specific errors: these may arise from the various        subsystems of the scanner used during exposure of the substrate,        in effect creating a scanner-specific fingerprint;    -   process induced wafer deformation: the various processes        performed on the substrates may deform the substrate or wafer;    -   illumination setting differences: these are caused by the        settings of the illumination system, such as the shape of the        aperture, lens actuator positioning, etc.;    -   heating effects—heating induced effects will differ between        various sub-fields of a substrate, in particular for substrates        wherein the various sub-fields include different types of        components or structures;    -   reticle writing errors: errors may be present already in the        patterning device due to limitations in its manufacture; and

topography variations: substrates may have topography (height)variations, in particular around the edges of wafers

The inventors have recognized that it is possible to reduce the overlayerror without using a higher-order correction model. By applying acorrection model to one or more specific portions of a particular field,rather than to the entirety of a particular field, overlay error can bereduced. These specific portions will in the following be referred to assub-fields (but may also in the art, e.g., be referred to as subzones).

For modeling the sub-fields, one may for example use only a standardcorrection model. Effectively, the parameters of the model are changedone or more times within each scanning operation, so that thecorrections are customized to the fingerprint of each part of the field.Thus, overlay error can be reduced without requiring use of the moreadvanced correction models. However, by using a standard correctionmodel in accordance with the method of the invention, the throughput ofwafers is not adversely impacted. Provided the patterning apparatusformed by projection system PS and associated positioning systems in theapparatus of FIG. 1 can be controlled to vary the model parameters fordifferent portions of each field, the new type of correction can beimplemented merely by suitable changes in the alignment and controlsoftware.

Modeling overlay error of individual sub-fields of a field can becarried out instead of modeling the overlay error of the field in itsentirety, or it can be modeled in addition to modeling the field in itsentirety. While the latter requires more processing time, since both thefield as well as the sub-fields within it are modeled, it allows for thecorrection of error sources which relate to a particular sub-field onlyas well as error sources which relate to the entirety of the field.Other combinations, such as modeling the entire field and only certainsub-fields, are of course possible.

With reference to FIG. 6, a lithographic method for correcting overlayerrors according to an embodiment of the present invention isillustrated. The reference numerals in this figure refer to thefollowing steps, each of which will be explained in more detail in thefollowing:

-   -   601: Exposing at least one field on a substrate;    -   602: Performing measurements on the field;    -   603: Determining sub-field;    -   604: Processing data relating to the sub-field to produce        sub-field correction information; and    -   605: Correcting exposure of the sub-field using the sub-field        correction information;

It is to be noted that, although the above steps are depicted in FIG. 6and discussed below in a particular order, some of these steps may beperformed in a different order, or may be performed simultaneously

In step 601, a lithographic exposure process is carried out on one ormore substrates using a scanner. The resulting exposed substrate willcontain overlay errors arising from one or several of the causesdescribed previously. The substrate can be a product substrate, or itcan be an initial “prototype” substrate made prior to start ofproduction. In step 602, measurements are performed at specific pointson the substrate(s). The number and distribution of measurement pointscan be varied in any suitable fashion. For example, measurement pointscan be arranged so as to be clustered around a particular area ofinterest, or they can be arranged in a grid pattern. In anotherembodiment, the measurement points may be randomly distributed. Themeasurements will reveal both inter-field fingerprints as well asintra-field fingerprints. In step 603, at least one sub-field isdefined. The sub-field can be defined in a number of ways, as will bediscussed in more detail below. In step 604, the obtained measurementresults are processed for each sub-field of the field, in order todetermine any corrections necessary to correct for any overlay errors.This is done by using a correction model as described above. In step605, in the exposure of further substrates, the exposure of a givensub-field is corrected based on the obtained correction information forthat sub-field, in addition to (or instead of) the corrections based onthe intra-field fingerprint modeled for the whole field. The exposureinformation is normally contained in the recipe data 306 described withreference to FIG. 3 above. As a result, the scanner is enabled tocontrol the exposure of a product substrate with greater accuracy thanknown.

Individual sub-fields can be defined in a number of different ways. Forexample, a sub-field can be defined by a user, either entirely manuallyor aided by measurement data. The user can, for example, define thesub-field by using a user interface on the lithographic apparatus or onthe supervisory control system or on a suitable remote device.

FIG. 7 shows an exemplary field 701 which is divided into a number ofequally sized sub-fields 702. Such a division of a field is useful ifthe field, for example, contains a number of equally sized and spacedproducts, product features, or product areas. However, sub-fields canequally well be defined so as to contain individual components orproducts which are not equal in size or distributed evenly across thefield. FIG. 7 shows a field 703 on which a product will be formed with anumber of different components occupying different product areas. As anexample, each field on the substrate may have a graphics processor insub-field 704, a processor core in each of sub-fields 705, a cache insub-field 706 and a system memory controller in sub-field 707. Eachsub-field is defined so as to contain one of these components. Bydefining sub-fields to hold one product each, the overlay error can becorrected individually for each product, even if the products are notevenly distributed or equal in size. This minimizes the deviationscaused by the standard correction model since the correction model canbe optimized for the part of the sub-field in which the product islocated, as discussed with reference to FIG. 5 above.

In order to further optimize the method, the definition of sub-fieldsmay also take additional factors into account, such as the location of aparticular field on the individual substrate. FIG. 8 illustrates anexemplary wafer 800, which is divided into a number of fields 802.Different fields will be used to illustrate different techniquespossible within the scope of the present disclosure. In a first field804, a sub-field has been defined, as explained above, so as to containa critical product or product area. The portion of the first field thatis outside the sub-field contains only less-critical products or productareas, which are tolerant of larger overlay areas. This approach usesthe standard correction model as described above, and is advantageous ifonly a single area of a field is intolerant of overlay error since itminimizes measurement and calculation time.

A second field 806 is divided into a number of sub-fields 808 which areequally spaced, although they could also be defined as described abovewith reference to FIG. 7. Whilst this implementation requires morecalculations, and hence more time to perform, than the implementation inthe first field 804, it reduces the overlay error for the whole field,even when using only a standard correction model. As such, such anapproach is advantageous if a field in its entirety is intolerant ofoverlay error, or if a field contains a number of products or productareas, each of which may be intolerant of overlay error.

A third field 810 of the wafer 800 is located at the edge of the wafer.The field is divided into a number of sub-fields in a similar manner tothe second field. However, since the field is located at the edge of thewafer, it contains a number of complete sub-fields 812 and an number ofincomplete sub-fields 814. Due to the proximity of the edge of thewafer, substrate-related deviations in such a field, and accordingly anysub-fields within, differ from deviations closer to the center of thewafer. In the past, such fields have not been used for products due tothe variation in deviation from more central fields. However, toincrease productivity, it would be advantageous if this space too isused. By dividing the third field into a number of sub-fields anddetermining the overlay error on an individual sub-field basis, it ispossible to utilize at least some of the sub-fields near the wafer edgefor products.

FIG. 9 illustrates the step of processing data relating to a sub-fieldof a particular exemplary implementation method described above withreference to FIG. 6 in more detail. In the this exemplaryimplementation, the sub-fields of the field are defined as rowstransverse to the scanning direction. The reference numerals in thisfigure refer to the following steps, each of which will be explained inmore detail in the following:

-   -   901: Obtaining intra-field fingerprint;    -   902: Performing simple intra-field model on entire fingerprint;    -   903: Performing simple intra-field on each row of fingerprint;    -   904: Adjusting parameters of actuators;

It is to be noted that, although the above steps are depicted in FIG. 9and discussed below in a particular order, some of these steps may beperformed in a different order, or may be performed simultaneously.

In step 901, measurement data relating to a particular field on thesubstrate are acquired. The measurement data typically contains datafrom a number of data sources, and can, for example, include (withoutlimitation): data relating to the scanner itself; earlier measurementdata (for example obtained from other substrates); or simulation data.Other data types which may be used include topography data or reticledata. In step 902, a linear intra-field correction model is applied tothe fingerprint of the entire field. As previously described, a linearcorrection model can comprise a number of parameters which define anumber of different parameters. In the present example, a correctionmodel will be described which uses six parameters, which together definetranslation, rotation and scaling of the ‘ideal’ grid, each in twodifferent dimensions (i.e. the x and y directions of a plane). In orderto decrease the overlay error, the six-parameter correction model willin step 903, in addition to step 902 and subsequently thereto, beapplied to at least one sub-field of the field fingerprint. It is to benoted that the sub-fields can be defined in any advantageous or suitablefashion. Advantageously, the sub-fields can be defined so as to containa portion of the field wherein critical components or products, whichare particularly sensitive to overlay error, are formed. Alternativelyor additionally, the sub-fields are defined in order to ensure thatparticular parameters and/or actuators of the lithographic apparatus canbe used to perform the corrections performed in step 903. As mentionedabove, in the present example the sub-fields of the field are defined asrows transverse to the scanning direction (i.e. in the y-direction). Onconclusion of step 903, a set of correction information has beenobtained, which can be used to determine adjustments to be made to oneor more of the actuators of the lithographic apparatus to control theexposure of the substrate. In step 904, the actual adjustments to theactuators are determined as a scanning operation is performed on thefield. In the present exemplary implementation, adjustments in thescanning direction are implemented by adjusting the speed of the reticlestage relative to the wafer stage. Adjustments transverse to thescanning direction can be implemented by adjusting one or more lensactuators of the lens system in the apparatus. Thus, withoutcomplicating the underlying correction model, different parameters canbe applied in the model, in different portions of the field.

In the above exemplary implementation, actuator adjustments areimplemented by adjusting two specific actuator parameters. Depending onthe specific type of apparatus used, the number and type of theactuators used to implement a specific parameter adjustment may vary.

Additionally, when the correction information has been obtained and thenecessary actuator adjustments have been determined, the responsefunction of individual actuators should be taken into account. Theactuator will not be able to (fully) reduce the overlay error if thebandwidth required to correct the measured deviations exceeds thebandwidth of the relevant actuator. Reconsideration of one or moreaspects of the adjustments and corrections may be performed until anoptimal recipe is found.

FIG. 10 shows a schematic illustration of the principle of the methodaccording to the invention. FIG. 10(a) shows an artificial intra-fieldfingerprint 1001 of a field. A number of measured positional deviations1002 are shown as vectors. For illustrative purposes only, the fieldfingerprint includes only positional deviations in the y-direction. Inreality, the deviations are, of course, not limited to deviations in asingle direction. In the example, the upper half of the field deviatesby −5 nm in the y-direction, and the lower half of the field deviates by5 nm in the y-direction. In reality, of course, the residual deviationswill not necessarily be such precise rounded numbers.

FIG. 10(b) shows the result obtained when a standard intra-fieldcorrection model with 6 parameters is applied to the complete sub-fieldfingerprint 1001 The parameters are labeled tx, ty, mx, my, rx, ry, andrefer, respectively, to the translation, magnification and rotation inthe x and y directions. The magnitude of the residual deviations 1003are reduced from 5.0 nm to 2.9 nm. In this example, the model works byreducing the magnification in the y-direction. Expressing magnificationin parts per million (ppm), in this example parameter my=−0.4ppm effectsthe correction. The remaining parameters of the model remain neutral,i.e. tx, ty, rx, ry, mx=0. The residual deviations 1003 are therebyreduced relative to the uncorrected deviations 1002, but are not reducedto zero.

FIG. 10(c) illustrates the same artificial field fingerprint 1001 asFIG. 10(a). However, the sub-field is in this figure divided into twosub-fields 1011 a and 1011 b, as indicated by the respective dashedlines), which cover the upper and lower halves of the fieldrespectively.

FIG. 10(d) shows the result obtained when the standard 6-parameter fieldcorrection model is applied separately to each of the sub-fields 1011 aand 1011 b. For sub-field 1011 a, fitting the correction model to themeasured data yields the following result: tx, mx, my, rx, ry=0, andty=5 nm, i.e. a translation in the y-direction of 5 nm. Similarly,fitting the model for sub-field 1011 b yields the result: tx, mx, my,rx, ry=0, and ty=−5 nm, i.e. a translation in the y-direction of −5 nm.As shown in FIG. 10(d), the residual deviations for both sub-fields canbe cancelled entirely, rather than being reduced. Thus, by dividing thefield into two sub-fields, the overlay error correction is improved overthe whole-field six-parameter correction model, but without having touse a more complex model than the six-parameter field correction model,and in certain instances with better accuracy than a more complex model.

With reference to FIG. 11, an exemplary embodiment of the processingmethod of FIG. 7 will now be discussed. FIG. 11(a) illustrates anexemplary measured intra-field fingerprint 1101. Specifically, thispresent example contains reticle writing errors, which result in astripe-like pattern with high spatial frequency in the Y direction. Asbefore, each measured deviation is represented in the illustration by avector 1102. From these measured deviations, it is possible to derive asix-parameter correction model in a conventional manner.

FIG. 11(b) illustrates the corresponding result from the six-parameterintra-field correction model 1104 when applied to the entirety of thefield shown in FIG. 11(a). As previously explained, it is unlikely thatthe residual deviation can be completely eliminated by using thestandard correction model. Indeed, in the present example, most of thedeviations remain uncorrected.

In order to further reduce the residual deviation, the field is dividedinto a number of sub-fields, one of which is highlighted by dashed line1110. Each of the sub-fields is defined as a single row of measurementpoints extending in the y-direction across the width of the field (i.e.in a direction transverse to the scanning direction). It will of coursebe appreciated that the sub-fields could be defined in a number of otherways, including but not limited to rows, diagonals, or other geometricshapes. However, what shapes can be applied may be limited bycapabilities of the control system. In an example where the parametersof the model can be varied during a scanning operation in they-direction, then setting the model parameters for regions other thanrows or stripes may be pointless in view of the control capabilitiesavailable for the subsequent exposures. In the present example, overlaycorrection parameters are calculated for each sub-field using only thetranslation parameters (tx, ty) of the correction model. It is to benoted that this is for illustrative purposes only, and that it ispossible to use any of the parameters of the model in any particularcombination.

FIG. 11(c) illustrates the results of applying the correction model toeach individual sub-field. Each vector 1112 represents the overlaycorrection parameters (tx, ty) for the corresponding sub-field (i.e.row) of the field shown in FIG. 11(a).

The resulting model parameters for each of the sub-fields shown in FIG.11(c) are subsequently used together with the whole-field correctionmodel 1104 to modify the settings of the corresponding actuators of thelithographical apparatus. In the present example, overlay correctionparameters contain only translation components, which can be implementedin the lithographic apparatus by adjusting the relative motion betweenthe wafer stage and the reticle stage during the scanning operation.

FIG. 12 shows the determined relative motion of the stages of thelithographical apparatus as a function of the scan position for the rowillustrated in FIG. 11(c). The first plot 1201 illustrates the relativemovement between the reticle and the substrate in the y-direction (i.e.in the scanning direction) needed to compensate for the residualdeviation in the y-direction. The second plot 1202 illustrates therelative movement between the reticle and the substrate in thex-direction needed to compensate for the residual deviation in thex-direction.

Some or all of the above-mentioned functions can be fully or partiallycarried out on or with the aid of a suitable user-interface. The userinterface can accept different types of input, either from other systemsor subsystems in the apparatus or external to the apparatus.Alternatively, the user can provide input directly into theuser-interface.

An example of an operation which can be carried out on a user-interfaceis the definition of the at least one sub-field. Determination of asub-field is application dependent, and forms part of a productsolution. As such, a sub-field definitions can be unique to a products,part of products, masks, layers or even entire families or types oftechnology. Such definitions can be entered automatically, for examplefrom a database, or can be dependent on measurement data or other datadetermined by the apparatus.

Further, the user can specify parameters or other considerations, whichmay be unique to a particular substrate, for example as part of adevelopment process. Considerations include, but are not limited to,minimizing overlay error in certain areas (such as critical areas),minimizing overlay error for an entire sub-field or field.

CONCLUSION

The method and associated inspection apparatus disclosed herein enableone or more of the following benefits.

This invention provides a high spatial frequency intra-field correctioncapability, so that more accurate correction is possible

Enables use of new models, for example wafer edge effect models thathave a large gradient

By modeling sub-zone separately potential crosstalk between modelparameters can be avoided, so that more accurate estimation ofparameters is possible. Therefore also supports root cause analysis.

The new concept can also be applied to CD control

The sub-field corrections can be done in APC control for varying part,and can be done in a feed forward way for the static part.

Although specific reference may be made in this disclosure to the use offocus monitoring and control arrangements in inspection apparatuses suchas scatterometers, it should be understood that the disclosedarrangements may have application in other types of functionalapparatuses, as mentioned already above.

Although specific reference may be made in this text to the use ofinspection apparatus in the manufacture of ICs, it should be understoodthat the inspection apparatus described herein may have otherapplications, such as the manufacture of integrated optical systems,guidance and detection patterns for magnetic domain memories, flat-paneldisplays, liquid-crystal displays (LCDs), thin film magnetic heads, etc.The skilled artisan will appreciate that, in the context of suchalternative applications, any use of the terms “wafer” or “die” hereinmay be considered as synonymous with the more general terms “substrate”or “target portion”, respectively.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. Furthermore, parts of the apparatus may beimplemented in the form of a computer program containing one or moresequences of machine-readable instructions describing a method asdisclosed above, or a data storage medium (e.g. semiconductor memory,magnetic or optical disk) having such a computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic method comprising: exposing a number of fields on asubstrate; obtaining data about a field; defining a sub-field of thefield based on the obtained data; processing data relating to thesubfield to produce sub-field correction information; and correctingexposure of the sub-field using the sub-field correction information. 2.A method according to claim 1, wherein the data obtained is afingerprint for the field.
 3. A method according to claim 2, wherein thesub-field is a line of data points in the fingerprint.
 4. A methodaccording to claim 1, wherein the data obtained further includestopography, layout, structure or simulation data.
 5. A method accordingto claim 1, wherein the data is obtained separately from exposing or atthe same time.
 6. A method according to claim 1, wherein exposinginvolves using a reticle, and the method further comprises obtainingdata about the reticle.
 7. A method according to claim 1, furthercomprising processing all or substantially all of the data obtained toproduce complete field correction information, and correcting exposureof the complete field using the complete field correction information.8. A method according to claim 1, wherein the processing comprisesapplying a model to the data and the correction information comprises aset of corrections from the model.
 9. A method according to claim 1,comprising: processing data relating to a number of sub-fields toproduce sub-field correction information for each sub-field; andcorrecting exposure of each sub-field using correction information forthat sub-field.
 10. A method according to claim 9, wherein exposure ofnumber of sub-fields is corrected at the same time or one after theother.
 11. A lithographic lithographic apparatus configured to performthe method according to claim
 1. 12. A non-transitory computer programproduct containing one or more sequences of machine-readableinstructions configured to control a lithographic apparatus system to:expose a number of fields on a substrate; obtain data about a field;define a sub-field of the field based on the obtained data; process datarelating to the sub-field to produce sub-field correction information;and correct exposure of the sub-field using the sub-field correctioninformation.
 13. A computer program product according to claim 12,wherein the computer program provides a user interface for use by anoperator in defining one or more sub-fields.
 14. A computer programproduct according to claim 13 wherein the user interface provides forthe operator to identify one or more portions of the field whereperformance of the exposure is to be optimized.
 15. A computer programproduct according to claim 13 wherein the user interface is arranged toconstrain choices of sub-fields in accordance with responses of specificactuators within the particular lithographic apparatus system.
 16. Acomputer program product according to claim 13, wherein the dataobtained is a fingerprint for the field.
 17. A computer program productaccording to claim 13, wherein the data obtained further includestopography, layout, structure or simulation data.
 18. A computer programproduct according to claim 13, wherein exposing involves using areticle, and the method further comprises obtaining data about thereticle.
 19. A computer program product according to claim 13, whereinthe instructions are further configured to cause the lithographicapparatus system to process all or substantially all of the dataobtained to produce complete field correction information, and correctexposure of the complete field using the complete field correctioninformation.
 20. A computer program product according to claim 13,wherein the instructions configured to cause the processing of the datacomprises instructions configured to cause application of a model to thedata and the correction information comprises a set of corrections fromthe model.